Lucas 25D4 40872 distributor users?

[dickie65]

I use a bog standard manual dizzy on my car, its in very good condition with decent gears and no spindle slop.
I hardly ever have to change the timing between road use sprinting or hillclimbing.
My engine works between 2000 rpm and 6500 rpm on the same amount of advance.
I recently had my car on a rolling road to get the SU set up and changing the timing made no difference to the power output !
Unless it was retarded too much.
The tuner said its down to the fact that side valve engines have such low compression.

My engine made 28 BHP after the needle and spring were modified.
Most Austin engines are in a low state of tune and usually operate between 800 Rpm and 4000 Rpm so should not need much adjustment of the timing.

I always though the vacuum take off retarded the ignition when the throttle was closed on tickover and the bob weights advanced it as the revs increase.
If you open the throttle you get less vacuum.
Those with vacuum wipers will know this.

[Ian Williams]

To clarify, I used the mechanical part of a 25D4 in a no vacuum Lucas body 23D I think from memory and I would have also made a few mods to the advance curve. I used this set up for many years successful racing and on more than one rolling road my car produced around 36 to 38bhp. Perhaps I could have done as well on circuit racing with a locked distributor but my special was also run on the road some of the time and I wanted to retain some flexibility in the engine. All I can say is it worked for me, and I probably ran the quickest non blown car that was raced here in NZ, I certainly never came up against a faster car other than a blown special that simply out dragged me on the straights.

[Chris KC]

Dickie65 that's nearly right - 

The "worst case" for ignition setting is low engine speed under high load; in a car with fixed timing this is the condition which determines the maximum advance which can be tolerated, and everything else is a compromise (unless the car only runs at 6000 rpm as on a race track).

Bob weights advance the ignition as engine speed increases, and retard it as engine speed decreases. 
For sporting use the revs are up and the ignition is advanced; when engine speed drops off e.g. slogging up a hill the ignition is backed off to prevent pre-ignition (which is a Bad Thing). A better compromise than fixed timing is arrived at, where ignition timing is adjusted with engine speed.

Vacuum advance is an added layer of sophistication, in that it senses whether the engine is under load or not.

Whereas a slow-running engine under load has to be retarded considerably to avoid pinking, the same engine running at the same speed on the flat with a mere whiff of throttle applied can tolerate - indeed would welcome - considerably more advance. Vacuum is essentially a function of throttle opening and a vacuum-controlled distributor - working alongside the bob weights - backs off the advance when the throttle is open and increases it when the throttle is closed. 

Obviously the two opposing mechanisms are tuned to give the best compromise - but still a compromise - which enables ignition advance with engine speed but moderates it according to the engine load as indicated by throttle position.

Pondering the above suggests to me that vacuum advance is of little interest to the go-faster crowd, but might improve running efficiency for those who spend a lot of time pottering along at low engine speed and barely-open throttle - a number of my continental holidays have been like that.